This invention relates to warewashers such as dishwashing machines, and more particularly to systems for chemically sanitizing dishes during the rinse cycle in lieu of thermal sanitization. The use of solutions such as sodium hypochlorite for chemical sanitization is known in the prior art. See, for example, U.S. Pat. Nos. 2,592,884, 2,592,885, 2,592,886, 3,044,092, 3,146,718, and 3,370,597, all of which are assigned to the assignee of the present invention. As more extensively explained in these references, the addition of predetermined small quantities of sanitizing liquid to the rinse water can provide a sanitizing bactericide equivalent to a high temperature rinse, providing an effective alternative and potential savings of energy where hot water is not readily available or economical.
The sanitizing solution is usually either injected into the wash chamber at full strength or carried into the chamber by a stream of water. When water carries the sanitizing solution, the water is usually a diverted portion of the fresh rinse water, as exemplified in the above-noted references. Typically, the diverted water passes through a venturi which aspirates the sanitizing solution into the water at the desired rate. The water then carries the sanitizing solution in diluted form into the dishwasher chamber.
Direct injection of additives can cause distinct problems. The most common sanitizing solutions are corrosive at full strength, and if permitted to seep directly into the wash chamber can cause serious corrosion. They tend to corrode their injectors and adjacent items, such as the wash tank, causing them to leak. The injectors may also leak when the solution dries or crystallizes to the extent that it interferes with the operation of the valves within the injector system. Such leakage allows the solution to drip or seep from the injector into the wash chamber, the concentrated solution then literally eating through the tank at that location, even when the tank is made of stainless steel. Experience has shown that such injectors therefore require frequent service.
A flowing stream of water for aspirating and transporting the sanitizing agent into the wash chamber in diluted form will substantially reduce the likelihood of corrosion within the chamber, but will create other undesirable side effects. Water supplies frequently contain minerals which in solution and as undissolved solids cause build-up of harmful line-clogging deposits, particularly when chemical reactions between the minerals in the water and the chemical sanitizing agent cause the minerals to precipitate from the solution. Formation of such scale also occurs when parts of the system dry out during idle periods.
The chemical reaction between hard water and the most commonly-used sanitizing agent, sodium hypochlorite, causes calcium and magnesium in the water to collect on adjacent parts. These deposits can block the proper operation of valves and clog the needle orifice of the water powered venturi which aspirates the sanitizing agent into the water stream, initially varying the predetermined quantity of agent injected and eventually making the system ineffective to perform its intended bacteria-killing function. The operator is seldom aware that the strength of the killer is diminishing unless the liquid supply lasts an inordinate amount of time, or a qualified sanitarian makes a concentration check. Thus, frequent servicing due to water mineral deposits at critical locations for metering the chemical solution is a problem with prior art water driven aspirators. Additionally, the flowing pressure of the rinse water varies considerably from location to location and even changes from time to time in the same line. Since the proportions of water and sanitizing agent are to be kept within close limits, a downward variation of agent to water can result in a total loss of sanitizing effect.
It is thus clear that both direct mechanical injection of the chemical sanitizing agent into the tank and water venturi injection into a flowing stream of water for subsequent injection into the tank are fraught with serious reliability problems. A need thus remains for a reliable, durable, and substantially service-free system for adding a liquid sanitizing agent to the rinse water of a dishwasher, in which the sanitizing agent can be supplied without contacting any moving parts, which will continue to operate without degradation of system performance and reliability caused by deposits from entrained minerals in the available water supply, and which will not be adversely affected by changes in flow pressure of the water supply.